Vertical integration of hybrid waveguide with controlled interlayer thickness

ABSTRACT

A silicon photonics device of hybrid waveguides having a coupling interlayer with an accurately controlled thickness and a method of making the same. The device includes a first plurality of Si waveguides formed in a SOI substrate and a first layer of SiO 2  overlying the first plurality of Si waveguides and a second plurality of Si 3 N 4  waveguides formed on the first layer of SiO 2 . At least one Si 3 N 4  waveguide is disposed partially overlapping with at least one of the first plurality Si waveguides in vertical direction separated by the first layer of SiO 2  with a thickness controlled no greater than 90 nm. The device includes a second layer of SiO 2  overlying the second plurality of Si 3 N 4  waveguides. The method of accurately controlling the coupling interlayer SiO 2  thickness includes a multilayer SiO 2 /Si 3 N 4 /SiO 2  hard mask process for SiO 2  etching and polishing as stopping and buffering layer as well as Si waveguide etching mask.

CROSS-REFERENCE TO RELATED APPLICATIONS

N/A

BACKGROUND OF THE INVENTION

The present disclosure is related to manufacture technique forintegrated silicon photonics device, and particularly to a verticallyintegrated hybrid waveguide with accurate interlayer thickness controland a method of making the same.

As science and technology are developed rapidly, processing speed andcapacity of the computer increase correspondingly. The informationtransmission or reception using the traditional cable is limited tolower bandwidth as compared to what optical fiber provides. In theadvent of Information Age, traditional electrical communication has beenlargely replaced by fiber-optic communication which provides much higherbandwidth and much longer distance transmission. An opticalcommunication system includes both electrical devices and opticaldevices, devices for converting electrical signal and optical signalback and forth, and devices for processing these signals. With theadvances of optical communication technology and applications driven bythe market demand on increasing bandwidth and decreasing packagefootprint, more intensive effort and progress have been seen in thedevelopment of electro-photonic integrated circuits onsilicon-on-insulator (SOI) substrate for forming those communicationdevices.

For example, SOI-based silicon photonics device containing Si waveguidecan monolithically be integrated with optically active devices such aselectro-optic modulators and Ge photo-detectors (PD). Due to highrefractive index contrast of Si vs. SiO₂, it enables manufacturing ofdensely integrated electronic-photonic components at low costs and highvolumes. Another advantage lies in good thermal conductivity of Si,which makes it suitable to fabricate thermally tunable photonic devices.However, for improving the performance with high tolerance onenvironmental temperature, susceptible to higher optical power, highoptical transparency, lower propagation loss, less dispersion, and lesssensitivity to waveguide shape, different material such as Si₃N₄ isintroduced into Si-based SOI photonics platform to enjoy both advantagesto Si and Si₃N₄ waveguide. It is desirable to have improved technique tointegrate both active and passive optical functionalities in a samephotonics chip. The hybrid waveguide which is made on aSi₃N₄-on-SOIplatform is a good candidate to design and fabricate good performance ofboth passive and active optical devices.

BRIEF SUMMARY OF THE INVENTION

The present disclosure is related to manufacture technique forintegrated silicon photonics device. More particularly, the inventionprovides a vertically integrated hybrid waveguide with accurateinterlayer thickness control and a method of making the same. In certainembodiments, the invention is applied for high speed opticalcommunication, though other applications are possible.

In modern electrical interconnect systems, high-speed serial links havereplaced parallel data buses, and serial link speed is rapidlyincreasing due to the evolution of CMOS technology. Internet bandwidthdoubles almost every two years following Moore's Law. But Moore's Law iscoming to an end in the next decade. Standard CMOS silicon transistorswill stop scaling around 5 nm. And the internet bandwidth increasing dueto process scaling will plateau. But Internet and mobile applicationscontinuously demand a huge amount of bandwidth for transferring photo,video, music, and other multimedia files. This disclosure describestechniques and methods to improve the communication bandwidth beyondMoore's law.

In an embodiment, the present invention provides a method for formingvertically integrated hybrid waveguides with interlayer thicknesscontrol. The method includes providing a silicon-on-insulator (SOI)substrate and forming a multi-layer hard mask comprising a siliconnitride layer overlying a first silicon oxide layer overlying the SOIsubstrate. Additionally, the method includes forming one or more siliconwaveguides in the SOI substrate, each silicon waveguide carrying themulti-layer hard mask on top. The method further includes forming adielectric layer to overlay the one or more silicon waveguides and coverany gaps separating the one or more silicon waveguides. Then, the methodincludes removing at least partially the dielectric layer to expose themulti-layer hard mask overlying at least one of the one or more siliconwaveguides. Furthermore, the method includes removing at least partiallythe multi-layer hard mask to leave substantially the first silicon oxidelayer overlying the at least one of the one or more silicon waveguides.The method also includes forming a second silicon oxide layer overlyingthe first silicon oxide layer such that total thickness of the firstsilicon oxide layer and the second silicon oxide layer is controlled tobe no greater than 90 nm across entire area of the SOI substrate.Moreover, the method includes forming one or more silicon nitridewaveguides. At least one of the one or more silicon nitride waveguidesoverlays the second silicon oxide layer at least vertically overlappingwith the at least one of the one or more silicon waveguides.

In an alternative embodiment, the present invention provides a siliconphotonics device with hybrid waveguides. The silicon photonics deviceincludes a silicon-on-insulator (SOI) substrate and a first plurality ofsilicon planar waveguides formed in the SOI substrate. Additionally, thesilicon photonics device includes a first layer of silicon oxide with athickness being controlled no greater than 90 nm across entire SOIsubstrate overlying the first plurality of silicon planar waveguides.Furthermore, the silicon photonics device includes a second plurality ofsilicon nitride planar waveguides formed on the first layer of siliconoxide. At least one of the second plurality of silicon nitride planarwaveguides is disposed in vertical direction at least partiallyoverlapping with at least one of the first plurality of silicon planarwaveguides. Moreover, the silicon photonics device includes a secondlayer of silicon oxide overlying all of the second plurality of siliconnitride planar waveguides.

Many benefits are provided with the improvement according to the presentinvention. In certain embodiments, the present invention provides. Thepresent invention achieves these benefits and others in the context ofbroadband communication technology. However, a further understanding ofthe nature and advantages of the present invention may be realized byreference to the latter portions of the specification and attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The following diagrams are merely examples, which should not undulylimit the scope of the claims herein. One of ordinary skill in the artwould recognize many other variations, modifications, and alternatives.It is also understood that the examples and embodiments described hereinare for illustrative purposes only and that various modifications orchanges in light thereof will be suggested to persons skilled in the artand are to be included within the spirit and purview of this process andscope of the appended claims.

FIG. 1 is a schematic sectional view of vertically integrated hybridSi₃N₄/Si waveguides in a Silicon-on-Insulator (SOI) substrate accordingto an embodiment of the present invention.

FIG. 2 is a schematic sectional view of a SOI substrate deposited withhard mask layers according to an embodiment of the present invention.

FIG. 3 is a schematic sectional view of silicon waveguides formed on theSOI substrate of FIG. 2 according to an embodiment of the presentinvention.

FIG. 4 is a schematic sectional view of additionally processed siliconwaveguides of FIG. 3 according to an embodiment of the presentinvention.

FIG. 5 is a schematic sectional view of a dielectric material overlaidall silicon waveguides of FIG. 4 according to an embodiment of thepresent invention.

FIG. 6 is a schematic sectional view of the silicon waveguides of FIG. 5with dielectric material overlayer being partially removed according toan embodiment of the present invention.

FIG. 7 is a schematic sectional view of the silicon waveguides of FIG. 5with a first planar overlayer surface according to an embodiment of thepresent invention.

FIG. 8 is a schematic sectional view of the silicon waveguides of FIG. 5with a second planar overlayer surface according to an embodiment of thepresent invention.

FIG. 9 is a schematic sectional view of the silicon waveguides of FIG. 5with a third planar overlayer surface according to an embodiment of thepresent invention.

FIG. 10 is a schematic sectional view of silicon nitride waveguidesformed on a third planar overlayer surface at least partially verticallyoverlapped with selected ones of silicon waveguides of FIG. 9 accordingto an embodiment of the present invention.

FIG. 11 is a flow chart illustrating a method for forming an insulatorinterlayer with controlled thickness for coupling vertically a Si₃N₄waveguide over a Si waveguide according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present disclosure is related to manufacture technique forintegrated silicon photonics device. More particularly, the inventionprovides a vertically integrated hybrid waveguide with accurateinterlayer thickness control and a method of making the same. In certainembodiments, the invention is applied for high speed opticalcommunication, though other applications are possible.

The following description is presented to enable one of ordinary skillin the art to make and use the invention and to incorporate it in thecontext of particular applications. Various modifications, as well as avariety of uses in different applications will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to a wide range of embodiments. Thus, the present inventionis not intended to be limited to the embodiments presented, but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

In the following detailed description, numerous specific details are setforth in order to provide a more thorough understanding of the presentinvention. However, it will be apparent to one skilled in the art thatthe present invention may be practiced without necessarily being limitedto these specific details. In other instances, well-known structures anddevices are shown in block diagram form, rather than in detail, in orderto avoid obscuring the present invention.

The reader's attention is directed to all papers and documents which arefiled concurrently with this specification and which are open to publicinspection with this specification, and the contents of all such papersand documents are incorporated herein by reference. All the featuresdisclosed in this specification, (including any accompanying claims,abstract, and drawings) may be replaced by alternative features servingthe same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

Furthermore, any element in a claim that does not explicitly state“means for” performing a specified function, or “step for” performing aspecific function, is not to be interpreted as a “means” or “step”clause as specified in 35 U.S.C. Section 112, Paragraph 6. Inparticular, the use of “step of” or “act of” in the Claims herein is notintended to invoke the provisions of 35 U.S.C. 112, Paragraph 6.

Please note, if used, the labels left, right, front, back, top, bottom,forward, reverse, clockwise and counter clockwise have been used forconvenience purposes only and are not intended to imply any particularfixed direction. Instead, they are used to reflect relative locationsand/or directions between various portions of an object.

Intensive effort and progress have been seen in the development ofphotonic integrated circuits in silicon-on-insulator (SOI). Using Si tomake photonic devices has many advantages. For example, Si waveguide canmonolithically integrate with optically active devices, e.g.,electro-optic modulators and Ge photo-detectors (PD). High refractiveindex contrast between Si and SiO₂ enables manufacturability of denselyintegrated electronic-photonic components at low costs and high volumes.Si also has good thermal conductivity, which makes it suitable tofabricate thermally tunable photonic devices. But for passive opticalcomponents, silicon nitride (Si₃N₄) waveguides with SiO₂ claddings canbe superior to Si waveguides in terms of less thermo-optic effects,greatly reduced optical nonlinearities, characteristics of has hightransparency into the visible wavelength range, and lower indexcontrast.

FIG. 1 is a schematic sectional view of vertically integrated hybridSi₃N₄/Si waveguides in a Silicon-on-Insulator (SOI) substrate accordingto an embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. As shown, one or more silicon nitride waveguides 110are vertically integrated with a silicon oxide interlayer of a thicknessd over corresponding one or more selected silicon waveguides 120 formedinto a SOI substrate 100 and buried by dielectric cladding material 130.Throughout the specification, a silicon nitride waveguide is referringto the Si₃N₄ waveguide and is made essentially by Si₃N₄ material. Asilicon waveguide is referred to the Si waveguide. The SOI substrate 100is provided with a Si layer formed on an insulator layer 102 on a Siwafer 101, where the Si layer on top has a predetermined thickness ofabout 220 nm with 20 nm margin. In some example, the Si layer thicknessis less than 300 nm, or is given a thickness suit for forming passivecomponents of a silicon photonics device 1000.

Referring to FIG. 1, the silicon photonics device 1000 includes severaltypes of planar waveguides: Si strip waveguide 10, Si₃N₄ waveguide 20,Hybrid Si₃N₄/Si waveguide, and Si rib waveguide 40. Waveguide 10, 20,and 30 are in standard planar strip shape of a common thickness equal tothe predetermined thickness of the Si layer provided in the SOIsubstrate 100 but can have various widths depending on layout forspecific applications. Waveguide 40 is in a rib shape with additional Simaterial being removed from at least part of the planar strip shapedwaveguide to have one or more extra steps of a height less than thepredetermined thickness of the Si layer in the SOI substrate 100.Waveguide 10, 20, and 40 are not hybrid type waveguide but a Si planarwaveguide or a Si₃N₄ planar waveguide alone. Waveguide 30 is a hybridone including a Si₃N₄ planar strip shaped waveguide 110 locatedvertically above and at least partially overlapped with a Si planarstrip waveguide 120 coupled by a dielectric interlayer with a thicknessof d. In an embodiment, the dielectric interlayer of thickness d is partof the insulator material 102, silicon oxide, of the SOI substrate 100.In another embodiment, as illustrated below in more details, each of theone or more Si planar waveguides 120 is formed from the Si layer of theSOI substrate 100. In a specific embodiment, the interlayer thickness dis controlled to be as thin as no greater than 90 nm to ensure a lowcoupling loss of <0.05 dB per transition. In another specificembodiment, interlayer thickness d is controlled to be at 80 nm with 20nm error margin. In later sections of the specification, the Si or Si₃N₄planar waveguide is simply referred as silicon (Si) waveguide or siliconnitride (Si₃N₄) waveguide.

Referring to FIG. 1 again, in an embodiment, the one or more Si₃N₄waveguides 110 have typical strip shapes with a common thickness ofabout 400 nm but with various widths, all substantially formed on a samelevel above the one or more Si waveguides 120 which have carious shapessuch as the four types mentioned above. For the hybrid Si₃N₄-on-Si type,in an example, the Si₃N₄ waveguide 110 has a planar strip shape with asmaller width than that of the Si waveguide 120 in strip shape locatedbelow the interlayer of thickness d. In another example, the Si₃N₄waveguide 110 above the interlayer of thickness d has a wider stripshape than that of the Si waveguide 120 below.

In a Si₃N₄-on-SOI photonics platform, the interlayer thickness d iscritical to ensure low loss coupling between the Si₃N₄ waveguide and theSi waveguide. The Si₃N₄ layer of the silicon photonics device 100 isused to make optical passive components, e.g., arrayed waveguide grating(AWG), spot-size convertor (SSC), multimode interferometer (MMI), delayline interferometer (DLI), and etc. Si layer of the silicon photonicsdevice 100, particularly, the rib shaped Si waveguide, is used to makecomponents for active optical devices, e.g., modulator, thermo-opticswitch, Ge detector. In those applications, 80 nm+/−10 nm SiO₂interlayer is a desired target thickness to ensure a low coupling lossof <0.05 dB per transition. However, in a mass production 0.18-μm CMOSfoundry process, it is difficult to control the SiO₂ interlayerthickness to meet the requirement of 80 nm+/−10 nm. Thus, this become adriving force to develop the following advanced technique forcontrolling the interlayer thickness to the desired target value forvertically integrated hybrid Si₃N₄/Si waveguides based on Si₃N₄-on-Siphotonics platform.

FIG. 2 through FIG. 10 below schematically depicts sequential steps of amethod for forming the vertically integrated hybrid Si₃N₄/Si waveguideswith accurately controlled interlayer thickness. FIG. 2 is a schematicsectional view of a SOI substrate deposited with hard mask layersaccording to an embodiment of the present invention. This diagram ismerely an example, which should not unduly limit the scope of theclaims. One of ordinary skill in the art would recognize manyvariations, alternatives, and modifications. As shown, a SOI substrate100 is provided with a Si layer 110 on insulator 102 overlying a Siwafer 101. Then, on entire top surface area of the Si layer 110, amultilayer hard mask 122 is formed by sequentially performing severaldepositions by PECVD. In an implementation, the multilayer hard mask 122includes a first silicon oxide film 102 a overlying entire area of Silayer 110 of the SOI substrate 100 followed by a formation of a siliconnitride film 120 a. In another implementation, the multilayer hard mask122 includes another silicon oxide film 102 b overlying the siliconnitride film 120 a. All these thin film depositions can be done underthe PECVD technique by with accurate thickness control of about <10%variation around a nominal thickness across entire area of the SOIsubstrate (e.g., entire wafer area). For example, the first siliconoxide film 102 a is targeted to a nominal thickness of 30 nm and thesilicon nitride film 120 a is targeted to a nominal thickness of 40 nm.Another silicon oxide film 102 b can be set to 30 nm as its targetthickness with 10% variation margin.

FIG. 3 is a schematic sectional view of silicon waveguides formed on theSOI substrate of FIG. 2 according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. As shown,multiple Si waveguides 111 are formed in strip shapes by performing Sietching processes based on predetermined pattern defined in themultilayer hard mask 122. Each strip shaped Si waveguide 111 stillcarries the original portion of the multilayer hard mask 122 on top ofthe Si layer 110 a of substantially the same thickness provided inoriginal SOI substrate 100. Each Si Waveguide 111 has various widthsdepending on the mask patterns and is separated from each other by a gapof various widths.

FIG. 4 is a schematic sectional view of additionally processed siliconwaveguides of FIG. 3 according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications. As shown, atleast one of the multiple strip shaped Si waveguides 111 is additionallysubjected to an etching process which removes addition portion of Si upto a partial thickness of the strip shaped Si waveguide 111 to form atleast one middle step in the strip. As a result, a rib Si waveguide 141is formed including a reduced Si layer 110 b made by a portion of Silayer retaining original thickness connected to another portion having apartial thickness. Of course, there can be other alternatives,variations, and modifications to the formation of Si waveguides in SOIsubstrate.

FIG. 5 is a schematic sectional view of a dielectric material overlaidall silicon waveguides of FIG. 4 according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asshown, all Si waveguides formed in previous steps, such as the hard maskcovered strip Si waveguides 111 and rib Si waveguide 141, are covered bya dielectric insulator material deposited using PECVD technique. In animplementation, this dielectric insulator material is substantially thesame insulator material 102 in the SOI substrate 100. In animplementation, this dielectric insulator material 102 is silicon oxide(SiO₂). The coverage of the dielectric material 102 is across entiresurface of the hard mask covered multiple Si waveguides 111 or 141 plusany gaps between them. In order to achieve full coverage, the thicknessof the dielectric material 102 can be up to nearly twice of the Si layerthickness. For example, the thickness of the dielectric materialoverlayer 102 above the hard mask covered Si waveguide 111 (or 141) with˜200 nm Si layer is about 400 nm (with 10% error margin). Of course,there can be other alternatives, variations, and modifications.

FIG. 6 is a schematic sectional view of the silicon waveguides of FIG. 5with dielectric material overlayer being partially removed according toan embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. As shown, a reverse etching process is applied toremove selected portions of the dielectric insulator material 102 basedon patterned etch masks. At least two etch masks are involved forperforming this reverse etching process. One etch mask is on top of hardmask covered strip Si waveguides 111 and another one is on rib Siwaveguide 141. Either etch mask allows the etching of the dielectricmaterial SiO₂ until it is substantially stopped by the silicon nitride(Si₃N₄) film 120 a in the multilayer hard mask 122 (FIG. 2). In anexample, for the dielectric material overlayer 102 of 400 nm±40 nm abovethe Si waveguide 110 a, the reverse etching removes the dielectricmaterial of about 330 nm±33 nm. After this step, the remaining film ontop of the Si waveguide 110 a is a thinned SiO₂ film of about 0˜66 nmoverlying originally placed Si₃N₄ film 120 a of about 25˜44 nm overlyingoriginally placed SiO₂ layer 102 a of about 30±3 nm during the formationof the multilayer hard mask 122. The silicon nitride film 120 a servesas an etch stopper for part of the SiO₂ reverse etching process whileleaving some non-etched portions of the SiO₂ overlayer film.

FIG. 7 is a schematic sectional view of the silicon waveguides of FIG. 5with a first planar overlayer surface according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asshown, the non-etched portions of SiO₂ overlayer film can be wiped outby performing a CMP process. In particular, the CMP process is preferredto be a short-time touch-up polishing process that polishes <40 nm SiO₂film. Normally, Si₃N₄ material is polished slower than SiO₂ material,e.g., half of SiO₂ polishing rate. Therefore, those non-etched portionsof SiO₂ overlayer 102 can be quickly removed including some Si₃N₄material to make the original silicon nitride film 120 a to a thinnersilicon nitride film 102 b overlying original first silicon oxide film102 a. After this CMP step, a planar overlayer surface 1021 is formed tobe substantially leveled with the silicon nitride film 120 b. Forexample, the silicon nitride film 120 b has a reduced thickness of 5˜27nm overlying the first silicon oxide film 102 a of about 30±3 nm.

FIG. 8 is a schematic sectional view of the silicon waveguides of FIG. 5with a second planar overlayer surface according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asshown, the silicon nitride film 120 b with reduced thickness is removedto form a planar surface 1022 to leave the first silicon oxide film 102a as last film on strip Si waveguide 110 a as well as on rib Siwaveguide 110 b. In a specific embodiment, the silicon nitride film isremoved using phosphoric acid (H₃PO₄) hot bath which specificallyremoves Si₃N₄ material but not the SiO₂ material. In an example, theremaining SiO₂ film over the strip Si waveguide 110 a is substantiallythe first silicon oxide film 102 a originally deposited by PECVD forforming the multilayer hard mask 122 on the Si layer 110 of the SOIsubstrate 100. Thus, this remaining SiO₂ film has accurately controlledthickness, for example, about 30±3 nm determined by previous PECVDprocess.

FIG. 9 is a schematic sectional view of the silicon waveguides of FIG. 5with a third planar overlayer surface according to an embodiment of thepresent invention. This diagram is merely an example, which should notunduly limit the scope of the claims. One of ordinary skill in the artwould recognize many variations, alternatives, and modifications. Asshown, additional silicon oxide material is deposited to increasethickness of SiO₂ overlayer film up to a target thickness d. As aresult, a new planar surface 1023 of the SiO₂ overlayer film is formed.Again PECVD is a preferred technique for depositing the silicon oxidematerial since by PECVD the deposited SiO₂ film thickness can be wellcontrolled within 10% error across entire area of the SOI substrate. Ina specific embodiment, about 50 nm of SiO₂ film is added on top of theoriginal first SiO₂ film of about 30±3 nm to form a final SiO₂ overlayerfilm having the target thickness d˜80 nm with 10% error margin overentire surface area of the SOI substrate. In an implementation, thisfinal SiO₂ overlayer film is utilized as a coupling interlayer between astrip Si waveguide 110 a beneath and another waveguide formed above.

FIG. 10 is a schematic sectional view of silicon nitride waveguidesformed on a third planar overlayer surface at least partially verticallyoverlapped with selected ones of silicon waveguides of FIG. 9 accordingto an embodiment of the present invention. This diagram is merely anexample, which should not unduly limit the scope of the claims. One ofordinary skill in the art would recognize many variations, alternatives,and modifications. As shown, multiple silicon nitride waveguides 120 areformed on the planar overlayer surface 1023 at various locations. In anembodiment, a silicon nitride waveguide 120 is formed at a location thatis not vertically overlapped with any silicon waveguide 110 a or 110 bbeneath the final SiO₂ overlayer film (which becomes natural extensionof original insulator material of the SOI substrate). In anotherembodiment, one or more silicon nitride waveguide 120 are formed atlocations that are at least partially overlapped with respective one ormore silicon waveguide 110 a in strip shape separated from each othervertically by the final SiO₂ overlayer film being as a couplinginterlayer having a thickness of d˜80 nm±10 nm at least across entireoverlapping area between the silicon nitride waveguide above and thesilicon waveguide below. The combination of these one or more siliconnitride waveguides 120 and the strip silicon waveguides 110 a separatedby the coupling interlayer of thickness d just form one or morevertically integrated hybrid Si₃N₄/Si waveguides.

Referring to FIG. 10, the silicon nitride waveguides 120 in these hybridSi₃N₄/Si waveguides are in typical strip shapes while their widths canbe varied to be either smaller or larger than corresponding widths ofstrip silicon waveguides beneath the coupling interlayer of thickness ofd. In the embodiment, the interlayer thickness of d is well controlledwithin a target value of about 80 nm with just 10% error margin by theseries of processes presented earlier from FIG. 2 through FIG. 9. Afterthe formation of these vertically integrated hybrid waveguides,additional dielectric insulator material, preferably the silicon oxidematerial 102 in the example, is deposited to cover entire surfaces ofthe multiple silicon nitride waveguides 120 plus any bare insulatorsurface area. As a result, the silicon photonics device 1000 depicted inFIG. 1 can be formed and used for integration with other passive/activeelectro-photonic circuits or devices for various optical communicationapplications.

In an alternative embodiment, the present invention provides a methodfor controlling thickness of an interlayer coupled between a siliconnitride waveguide and a silicon waveguide in Si₃N₄-on-Si platform. FIG.11 is a flow chart illustrating a method for forming an insulatorinterlayer with controlled thickness for coupling vertically a Si₃N₄waveguide over a Si waveguide according to an embodiment of the presentinvention. This diagram is merely an example, which should not undulylimit the scope of the claims. One of ordinary skill in the art wouldrecognize many variations, alternatives, and modifications in one ormore processes of the method. For example, one or more processes may beskipped or combined with another process. One or more processes may bereplaced by another process or switched in order. Additional processesmay be inserted in the sequential steps as shown. In a specificembodiment, the process flow includes steps that are substantiallyillustrated in FIG. 2 through FIG. 10 in above sections of thespecification.

Referring to FIG. 11, the method 2000 for forming an insulatorinterlayer with controlled thickness for coupling vertically a Si₃N₄waveguide over a Si waveguide is substantially compatible with standardCMOS technology based on a SOI substrate. A step 2010 is for forming amultilayer hard mask including a first SiO₂ film over a Si₃N₄ film overa second SiO₂ film stacked on the SOI substrate. These films are formedby deposition with respective film thickness being well controlledwithin 10% error. For example, state-of-art PECVD technique has beendeveloped to have corresponding deposition rate control to achieve thedesired film thickness control. In an example, a 300 Å-SiO₂/400Å-Si₃N₄/300 Å-SiO₂ multilayer hard mask is formed.

Referring to FIG. 11 again, the method 2000 includes a step 2015 forforming multiple Si waveguides separately disposed in the SOI substrate.This step can be done by conventional techniques like lithography,patterning, etching, etc. The Si waveguides as formed can be in typicalstrip shapes having various widths but substantially a same thickness asdefined by original Si layer over an insulator layer provided in the SOIsubstrate. For example, the Si layer of the SOI substrate is about 2000Å to 3000 Å. The insulator layer of the SOI substrate is made typicallyby silicon oxide, SiO₂, with a thickness of 2˜3 μm. Additional processcan be performed to further modify the shape of the Si waveguides. Forexample, extra Si etching can be performed on part of a strip Siwaveguide to reduce its thickness to partial thickness of original Silayer. The strip shape Si waveguide may be transformed to a rib shape orother shapes, although all Si waveguides are substantially still in asame height level relative to the SOI substrate.

The method 2000 additionally include a step 2020 for forming a thickSiO₂ film overlying the Si waveguides formed in earlier steps, toessentially embed all the Si waveguides within the same insulatormaterial as that in the SOI substrate. For example, 4000 Å thick SiO₂film deposition is performed by PECVD. In a specific embodiment, thedeposition thickness of SiO₂ film is 4000 Å±400 Å, with an assumption of±10% of total thickness variation which is readily achievable for PECVDdeposition.

Further, the method 2000 includes a step 2025 for removing partially thethick SiO₂ film on top of each of the multiple Si waveguides down to alevel below the top SiO₂ film of the multilayer hard mask. In anembodiment, this step is a reverse etching process for selectivelyremoving SiO₂ material with the Si₃N₄ film of the multilayer hard maskbeing served as an etch stopper on top of each of the multiple Siwaveguides. SiO₂ dry etching recipe with selectivity ratio of 5-10relative to Si₃N₄ can be developed. Two types of etch masks are involvedin this etching process. One is for Si strip waveguide reverse andanother is for Si rib waveguide reverse. Once the SiO₂ film above theSi₃N₄ film is substantially etched, the etching process is stopped. Ofcourse, this etch process may leave a plurality of non-etched portionsof SiO₂ material with reduced thickness (e.g, less than 400 Å) from thethink SiO₂ film formed in step 2020.

Furthermore, the method 2000 includes a step 2030 for cleaning up theremaining un-etched portions using chemical-mechanical polishing (CMP)process to form a planar surface. The CMP process is preferred to be ashort-time touch-up polishing technique that is designed to remove apre-set thickness (e.g., <400 Å) of SiO₂ layer. Again, the Si₃N₄ film ofpreviously formed in the multilayer hard mask serves as a stopping orbuffering material to make the planar surface substantially leveled withthe hard Si₃N₄ film. In this process, a recipe with selectivity ratio of2-3 times for SiO₂ CMP versus Si₃N₄ CMP is used, which is readilyavailable in state-of-art CMP equipment. Due to the much slowed CMP ratefor Si₃N₄ material, after the polishing step, the remaining Si₃N₄ filmon top of the Si waveguides may still have a thickness of about 50˜270 Å(out of the original ˜400 Å). Up to this step, the Si₃N₄ film is thestopping/buffering material for both SiO₂ reverse etching and CMP witha<400 Å SiO₂ polishing target, making sure that Si₃N₄ film is still, atleast partially, left after CMP process and the bottom 300 Å-SiO₂ filmis not touched. Finally, this ensures the dielectric thickness on top ofSi waveguide is controllable.

The method 2000 further includes a step 2035 for forming a second planarsurface substantially level with the bottom SiO₂ film with the Si₃N₄film being selectively removed. In a specific embodiment, hot H₃PO₄ bathprocess is applied to selectively remove the remaining Si₃N₄ film of50˜270 Å, while leaving the bottom 300 Å-SiO₂ film on top of the Siwaveguides. These SiO₂ films over each Si waveguide are substantiallyleveled with rest of SiO₂ material to share a common planar surfaceacross entire SOI substrate, which retain the original uniformityprovided by deposition process for forming the bottom 300 Å-SiO₂ film ofthe multilayer hard mask on the SOI substrate.

Moreover, the method 2000 includes a step 2040 for forming a thirdplanar surface by adding SiO₂ material overlying the bottom SiO₂ filmsuch that total SiO₂ film has a desired thickness above each Siwaveguide. Adding SiO₂ material is done by PECVD, which is much morecontrollable to give an accurate film thickness for the final SiO₂ filmover the Si waveguide than using any of those etch or CMP techniquesshown in earlier steps. For example, provided that a final SiO₂interlayer thickness of 800 Å is a target thickness, additional 500 ÅSiO₂ film is deposited with good thickness control over the original 300Å-SiO₂ film.

The method 2000 then includes a step 2045 for forming one or moresilicon nitride (Si₃N₄) waveguides overlying the third planar surface(of SiO₂ film) with at least an interlayer above selected one or more Siwaveguides with a thickness being well controlled within a desired valueacross entire surface area of the third planar surface. Adding Si₃N₄material to form the Si₃N₄ waveguide in (for example) strip shape orother shapes can be done by following conventional methods ofdeposition, patterning, etching under standard CMOS technology, underthe condition that the interlayer with the thickness is well controlledto the desired value or smaller for vertically coupling the Si₃N₄waveguide with the Si waveguide underneath has been formed based on themethod 2000. Finally, the method 2000 include a step 2050 for depositingSiO₂ film to bury entire Si₃N₄ waveguides vertically coupled with the Siwaveguides via a coupling interlayer in desired thin thickness (of atleast 80 nm±10 nm) to ensure a low coupling loss of <0.05 dB pertransition of light through these Si₃N₄-on-Si hybrid waveguides.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A method for forming vertically integrated hybridwaveguides with interlayer thickness control, the method comprisingmultiple steps in a sequential order of: (1) providing asilicon-on-insulator (SOI) substrate; (2) forming a multi-layer hardmask overlying the SOI substrate, the multi-layer hard mask comprising atop silicon oxide layer overlying a silicon nitride layer overlying afirst silicon oxide layer; (3) forming one or more silicon waveguidepatterns in the SOI substrate, each silicon waveguide pattern carryingthe multi-layer hard mask on top of at least one silicon waveguide; (4)forming a dielectric layer to overlay the one or more silicon waveguidepatterns and cover any gaps separating the one or more silicon waveguidepatterns; (5) removing at least partially the dielectric layer to exposethe multi-layer hard mask overlying the at least one silicon waveguide;(6) removing at least partially the multi-layer hard mask to leavesubstantially the first silicon oxide layer overlying the at least onesilicon waveguide; (7) forming a second silicon oxide layer overlyingthe first silicon oxide layer such that total thickness of the firstsilicon oxide layer and the second silicon oxide layer is controlled tobe no greater than 90 nm across entire area of the SOI substrate; (8)forming one or more silicon nitride waveguides, at least one of the oneor more silicon nitride waveguides overlying the second silicon oxidelayer at least vertically overlapping with the at least one siliconwaveguide.
 2. The method of claim 1 wherein forming the multi-layer hardmask comprises depositing by PECVD with film thickness control of about10% margin to sequentially form a top silicon oxide layer of 30 nmoverlying the silicon nitride layer of 40 nm or less overlying the firstsilicon oxide layer of 30 nm or less.
 3. The method of claim 1 whereinforming the multi-layer hard mask comprises patterning the multi-layerhard mask to define strip shapes for the one or more silicon waveguidepatterns.
 4. The method of claim 3 wherein forming one or more siliconwaveguide patterns comprises etching through a total thickness ofsilicon in the SOI substrate based on the strip shapes defined by themulti-layer hard mask with the top silicon oxide layer as an etchingmask.
 5. The method of claim 4 wherein forming one or more siliconwaveguide patterns comprises further etching partially through the totalthickness of at least one of the one or more silicon waveguide patternsin strip shape to form a silicon rib waveguide with at least one extrastep in the strip shape.
 6. The method of claim 1 wherein forming adielectric layer comprises depositing silicon oxide material up to athickness about twice of thickness of silicon in the SOI substrate. 7.The method of claim 1 wherein removing at least partially the dielectriclayer comprises applying a patterned etch mask and performing reverseetching to remove at least a portion of the dielectric layer with thetop silicon oxide layer of the multi-layer hard mask as an etch-stopperon top of each of the one or more silicon waveguides.
 8. The method ofclaim 7 further comprising removing remaining portions of the dielectriclayer by touch-up CMP polishing with the silicon nitride layer of themulti-layer hard mask as a stopper to form a planar surfacesubstantially leveling with the silicon nitride layer of the multi-layerhard mask.
 9. The method of claim 1 wherein removing at least partiallythe multi-layer hard mask comprises using phosphoric acid (H₃PO₄) hotbath to remove the silicon nitride layer while keeping substantially thefirst silicon oxide layer of the multi-layer hard mask.
 10. The methodof claim 1 wherein forming a second silicon oxide layer comprises usingPECVD process to deposit silicon oxide material with thickness controlof 10% margin to ensure that the total thickness of silicon oxidematerial above the one or more silicon waveguides is about 80 nm±10 nm.11. The method of claim 1 wherein forming one or more silicon nitridewaveguides comprises forming strip shaped silicon nitride waveguideswith a thickness of about 400 nm.
 12. The method of claim 1 wherein theone or more silicon nitride waveguides comprises at least one siliconnitride waveguide disposed not overlapped with any one of the one ormore silicon waveguides.
 13. The method of claim 1 further comprisingforming a dielectric material to fully embed the one or more siliconnitride waveguides partially over the one or more silicon waveguides inthe SOI substrate.